The present invention relates to an ion source, a mass spectrometer and mass spectrometry using the same, and an instrumental system using the same or a monitor using the ion source.
The minor constituents in air or liquid have so far been detected with high sensitivity by ionizing the sample to be measured, and detecting the ions on a mass spectrometer.
The atmospheric pressure chemical ionization method using corona discharge is one of the methods of ionizing the collected sample. In this method, as disclosed in JP-A-51-8996, a high voltage is applied, and the sample is introduced into the corona discharge region generated at the tip of a needle electrode, and ionized. When the negative ions are measured by use of the atmospheric pressure chemical ionization method, as disclosed in JP-A-2001-93461 and Japanese Patent Application No. 2000-247937, the sample gas introduced into the corona discharge region is forced to flow in a direction different from the direction of the ions, thereby enabling the minor constituents of the sample gas to be detected with high sensitivity.
However, in the prior art described in JP-A-51-8996, JP-A-2001-93461 and Japanese Patent Application No. 2000-247937, there is no description of how the sample should be introduced into the ion source in order that both positive ions and negative ions can be measured with high sensitivity.
Accordingly, it is an object of the invention to provide an ion source using corona discharge and a system using the same, in order to measure both positive ions and negative ions with high sensitivity.
When the minor constituents of air are ionized by the atmospheric pressure chemical ionization method using corona discharge, the following reactions can be considered to occur. To the needle electrode where corona discharge is to be caused, is applied a positive high voltage when the sample is to be positively ionized, or a negative high voltage when the sample is to be negatively ionized.
(Positive ionization)
N2+exe2x88x92xe2x86x92N2++2exe2x88x92
(Positive corona discharge)
N2++2N2xe2x86x92N4++N2
N4++Mxe2x86x92M++2N2
First, N2 molecules of air are non-selectively ionized by corona discharge, and then the electric charges are selectively shifted to the molecules of low-ionization energy by ion molecule reaction, thus producing ionized molecules M that are to be measured.
When the sample gas contains moisture, water cluster ions are produced as given below, thereby suppressing the reaction for producing the above M+ ions, with the result that the sensitivity is lowered.
N4++H2Oxe2x86x92H2O++2N2
H2O++H2Oxe2x86x92H3O++OH
H+(H2O)nxe2x88x921+H2O+N2xe2x86x92H+(H2O)n+N2
In order to prevent this suppression, a cooler is provided to remove the moisture from the sample gas before the sample gas is introduced into the ion source, so that the sensitivity can be improved. When the sample gas has low volatility enough to adhere to the inner wall of the cooler, the cooler is not used, and instead the temperature of the ion source and the first ion sampling aperture is raised to suppress the clustering of water so that the sensitivity can be improved.
(Negative ionization)
O2+exe2x88x92xe2x86x92O2xe2x88x92
(Negative corona discharge)
O2+N2xe2x86x922NO
(Negative corona discharge)
O2xe2x88x92+NOxe2x86x92NO3xe2x88x92
O2xe2x88x92+Mxe2x86x92(M-H)xe2x88x92+HO2
where (M-H)xe2x88x92 is the negative ion of M with a proton removed. In the case of negative ionization, molecules M are ionized through ions O2xe2x88x92 resulting from non-selective ionization by corona discharge. In this case, the intermediate NO produced at the same time easily changes to NO331  , and is observed as strong ions. Since NO3xe2x88x92 has a high degree of acidity, it often does not react with molecules M. Therefore, when the concentration of N2 is much higher than M, the NO3xe2x88x92 is nearly always observed, but (M-H)xe2x88x92 is less observed. In order to increase the efficiency of the production of (M-H)xe2x88x92, it is necessary that the direction in which O3xe2x88x92 ion is moved by the electric field be made different from the direction in which the intermediate NO moves together with the flow, and that the time for which the intermediate exists in the corona discharge region be decreased as much as possible. In other words, when the corona discharge is caused at the tip of the needle electrode by the application of a high voltage, the direction in which the needle electrode is connected to a partition wall having an opening through which the generated ions are introduced into the mass spectrometry portion, or the direction in which the ions are extracted from the discharge region, is made different from the direction of the flow of the sample gas, thereby greatly increasing the ionization efficiency of the sample.
In the case of positive ionization, since the charge of ion N2+ is never taken by the produced intermediate. Therefore, in order to efficiently detect the ions of the desired constituent, it is necessary that the directions in which the ions and the sample gas are moved be made the same so that the sample gas flow does not interfere with the ion movement.